Method of electroplating on aluminum surfaces



United States Patent Ofilice 3,075,894 Patented Jan. 29, 1953 3,975,894 NEH-16D 6F ELECTRGPLATING 6N ALUMINUM SURFACE?) Herbert E. Ricks, Forest Hills, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. Filed Jan. 23, 1959, Ser. No. 788,503

2 Ciaims. (Cl. 20433) This invention relates to a method of electroplating on aluminum surfaces, and in particular to a method of providing on the surface of an aluminum article a sound adherent coating of another metal such as silver.

The present trend in the electrical industry is towards the substitution of aluminum for copper for many applications. In particular, aluminum has been found highly satisfactory as a substitute for copper in .the manufacture of bus bars.

Satisfactory electrical joints or connections are difficult to make to aluminum members owing to the presence of the thin oxide film on the aluminum member which oxide film has a high electrical resistance. Therefore, in order to satisfactorily employ aluminum in most electrical applications, it has been found to be necessary to remove the oxide film and provide on the oxide-free aluminum surface a coating of a metal such as silver in order to make possible joints or connections having low contact resistance.

It has also been found to be necessary to first coat the surface of the aluminum member with a thin coating of zinc as a base for the ultimate silver plate, if satisfactory adhesion of the silver plate is to be obtained. This is usually accomplished by preplating the surface of the aluminum member with a layer of zinc from a suitable zincate plating bath.

The known zincate treatments have not been entirely satisfactory in that they are time consuming, cumbersome and expensive. In many of the known zincate treatments, as many as eight or ten individual critical and time consuming steps are involved before a satisfactory zinc coating is obtained on the aluminum surface for the subsequent deposition of the silver coating.

Attempts have been made to eliminate a substantial number of the critical steps involved in the prior art methods. These attempts have not proved to be entirely satisfactory owing mainly to the fact that consistently good silver plating results have not been obtained. Thus, for example, when the silver plated aluminum members are subjected to elevated temperatures such as during the deposition of molten solder thereon, or when employed in electrical applications where a current flows between the aluminum and the silver coating, blistering of the silver plate often occurs. Such blistering is objectionable in that it results in higher electrical resistance between the silver and zinc coating.

The object of this invention is to provide an improved and highly satisfactory method of providing on the surface of an aluminum member a silver plate which method comprises the steps of anodically etching the aluminum article in a bath comprising a mineral acid selected from the group consisting of sulfuric acid and nitric acid, applying to the etched aluminum member a coating of zinc, and then depositing thereon a silver plate.

Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and the objects of this invention reference is made to the following detailed description.

In accordance wtih this invention there is provided a method for applying highly satisfactory coatings of silver on aluminum members, which silver coated or plated aluminum members are particularly adapted for use in electrical applications.

The method of this invention involves substantially fewer steps than the prior art methods, and the method provides on the surface of the aluminum member a highly satisfactory silver plate or coating with consistently good results.

Briefly, the method of this invention involves anodically etching the aluminum member in a mineral acid electrolyte selected from the group consisting of sulfuric acid and nitric acid in such a manner as to remove the aluminum oxide present on the surface of the member. The aluminum member is then thoroughly rinsed in Water and immersed in a ziacate bath for a period of time suflicient to provide thereon a satisfactory coating of zinc. The aluminum member with the applied zinc coating is then rinsed in water and a silver strike is applied to the zinc coating followed by silver plating to desired thickness.

The etching and preparation of the aluminum surface prior to application of zinc coating is accomplished by making the aluminum article the anode in an electrolytic cell containing an electrolyte comprising from about 20% to by weight of a mineral acid selected from the group consisting of 100% sulfuric acid, 100% nitric acid and mixtures thereof and the balance water. For the purposes of this invention an electrolyte comprising -rom about 40% to 50% by Weight of sulfuric acid and/or nitric acid is preferred. A direct current sufficient to provide a current density of from about 50 to amperes per square foot is employed at a potential of from about 6 to 10 volts until the desired etching is accomplished, the electrolyte being maintained at room temperature. Usually a period of time from about 5 to 15 seconds is satisfactory to accomplish removal of the thin oxide film under the conditions specified. By this procedure substantially all the oxide film present on the surface of the aluminum article is removed.

The acid treatment employed in the method of this invention is a critical and important step therein. The use of prior art caustic etching solutions, such as sodium hydroxide result in the formation of compounds such as aluminum hydroxide and sodium alumina'te which compounds adhere to the surface of the aluminum. These compounds are extremely difficult to remove from the surface of the aluminum and as a result numerous rinse baths and time consuming rinse steps are required to satisfactorily remove these compounds from the surface of the aluminum. Any residual alkali present on the surface of the aluminum article Will cause the subsequently applied silver plate to blister when the plated aluminum members are subjected to elevated temperatures.

The acid solution employed herein for oxide removal is a free rinsing type solution which assists in removing substantially all harmful residue which may be present on the surface of the aluminum article. The acid etching treatment, followed by a single rinse in clean water provides an aluminum surface substantially free from oxide film and also removes any micro constituents which may interfere with the formation of a satisfactory zinc layer.

The etched aluminum article is immediately immersed in a suitable zincate solution for a period of from about 15 seconds to two minutes at room temperature until a layer of zinc of desired thickness has been applied thereto. Conventional zincate baths such as, for example, an aqueous solution of alkali metal zincate, may be employed in this step of the invention.

An example of a satisfactory alkali metal zincate bath that maybe employed in this invention is one that is made up with about grams of zinc oxide and about 400 grams of caustic soda per liter of bath, the balance being substantially all water. In practice, the zincate bath may be varied widely as to the proportions and concentrations of its components. The amount of zinc oxide may be much lower than 100 grams per liter of bath, but the ratio of caustic soda to zinc oxide should be between about 3:1 to 8:1. The bath may be made up with equivalent amounts of other zinc salts and other caustic alkalies. Other suitable zincate baths are well known in the art.

The coating of zinc thus obtained is a non-porous coating of such a character that the subsequently applied silver plate is of high quality and adheres extremely well to the underlying surface without blistering at all temperatures up to a melting point of the aluminum article.

After the immersion in the alkali zincate solution the aluminum article with the applied zinc coating is rinsed in clean water, as is conventional after such zincating treatment.

Thereafter a silver strike is applied to the zinc coated aluminum article. The application of the silver strike is an essential and important step in the method of this invention. It has been determined that this step is essential to secure a sound silver plate on the aluminum member that will not blister when subjected to elevated temperatures. Blistering of the silver plate results in higher electrical resistance between the silver and zinc coating.

Electrolytic baths and conditions for the application of .silver strikes are well known in the art.

An example of a suitable silver-strike bath is one composed of from about 1 gram to 5 grams of silver cyanide and from about 65 to 90 grams of sodium cyanide per liter of bath, the balance being substantially all water. The bath is maintained at a temperature of from about 70 F. to 85 F. during use. Direct current sufficient to provide a current density of from about 15 to 25 amperes per square foot is employed. Plating time required will vary from about 6 to 10 seconds.

The silver plate of desired thickness is then plated onto the aluminum member. Well known silver plating solutions may be employed for this purpose. It is preferred however, that the silver plating solution employed and the conditions employed for its deposition be such that a hard dense silver plate be applied. A suitable silver plating solution for depositing the desired silver plate is a water solution which comprises about 30 grams of silver cyanide, 55.5 grams of potassium cyanide, 45 grams of potassium carbonate and 41.3 grams of free potassium cyanide per liter of solution with the solution operated at a temperature of from about 90 F. to 110 F. The desired physical properties of the silver plate are best obtained at current densities of from about to 25 amperes per square foot. Adequate solution agitation and filtration should be employed in carrying out this step of this invention.

High speed silver plating methods may be employed if desired. Satisfactory silver plating solutions and plating conditions are well known in the art.

Silver plated articles of this invention are characterized by hard dense bright silver plate which is relatively nonporous and which adheres to the aluminum article at all temperatures up to the melting point of the aluminum. The method of this invention has produced silver plated aluminum articles with consistently good results.

The following example is illustrative of the method of this invention:

Example I An aluminum article is preliminarily cleaned in a water solution comprising about 60 grams of sodium hydroxide .per liter of solution. The article is then imersed in an electrolyte bath consisting of, by weight, 42% of 96% sulfuric acid and 58% water. The bath is operated at room temperature conditions. The aluminum article 18 made the anode in this bath and the anodic etching treat ment of the article at a current density of about 50 amperes per square foot for about 5 seconds is carried out.

The treated article is then rinsed in clean running water for about 45 seconds. A zinc coating is then applied by immersing the aluminum article in a ziucating bath comprising about 100 grams of zinc oxide and about 400 grams of sodium hydroxide per liter of bath, the balance being water for about 15 seconds. The aluminum article With the applied zinc coating is then rinsed in clean water for about 45 seconds.

A silver strike is applied to the zinc coated article by immersing the article in an electrolyte comprising about '1 gram of silver cyanide and grams of sodium cyanide per liter of bath, the balance being water. The zinc coated aluminum article is made the cathode on this bath and a direct current sufficient to provide a current density of about 15 amperes per square foot is employed for about 15 seconds to produce the silver strike on the zinc coating.

The treated aluminum article is then made cathodic in a cell containing an electrolyte comprising about 31 grams of silver cyanide, about 56 grams of potassium cyanide, about 45 grams of potassium carbonate and about 42 grams of free potassium cyanide per liter of solution, the balance being water. This bath is employed at about 80 F. and a current density of about 20 amperes per square foot is impressed on the cell for about 4 minutes. The silver plated aluminum bar is then rinsed in clean running water and dried. The applied silver plate has a thickness of about .0002 inch.

Molten solder at a temperature of about 300 C. was applied to the silver plated aluminum article and the applied silver plate did not blister.

The term aluminum as used herein and in the appended claims is intended to cover not only pure or commercially pure aluminum, but also various alloys of aluminum. Examples or" aluminum alloys which may be successfully processed by employing the method of this invention include an alloy of aluminum containing 1.2% manganese; an alloy containing about 1% magnesium, about 0.6% silicon, about 0.25% copper and about 0.25% chromium.

While this invention is particularly concerned with the application of electro-deposited silver coatings to aluminum members for electrical applications, it is to be understood that the invention is not necessarily restricted thereto and can be employed satisfactorily for the application of electro-deposited coating of copper, cadmium, tin, or multiple plates of copper and silver, cadmium and tin, and the like.

It will be apparent to those skilled in the art that the method of this invention results in an elimination of a substantial number of the time consuming steps employed in the prior art, and produces a silver plated aluminum member of substantially equal quality. The method of this invention now makes it economically feasible to spot plate aluminum bus bars with the attendant savings in material cost. This was not possible by the time consuming prior art methods, owing to the fact that the expense of the necessary handling equipment for spot plating far outweighed the cost savings resulting from spot plating the bus bars.

it will be understood that the above examples and description are illustrative of this invention and not in limitation thereof.

I claim as my invention:

1. In the method of silver plating an aluminum article the steps which comprise immersing the aluminum article in an electrolytic bath consisting essentially of 20% to 60%, by weight, of a mineral acid of concentration selected from the group consisting of sulfuric acid and nitric acid and the balance water, anodically etching the immersed aluminum article by passing therethrough a direct current of sufficient density to provide a current density of from about 50 to 75 amperes per square foot for about 5 to 15 seconds, rinsing the etched aluminum article in clean water, immersing the etched aluminum article in an aqueous alkali metal zincate solution for from about 15 seconds to 2 minutes to deposit thereon a substantially non-porous layer of zinc, rinsing the zinc coated article in clean water, electrodepositing a silver strike over the applied zinc layer from a strike bath essentially composed of an aqueous solution silver cyanide and sodium cyanide, said silver strike bath consisting essentially of from about 1 gram to grams of silver cyanide and from about 65 to 90 grams of sodium cyanide per liter of bath, the balance being water, and electrodepositing a silver plate over said silver strike whereby there is produced an aluminum article having a highly adherent silver plate applied thereto.

2. In the method of silver plating an aluminum article the steps which comprise immersing the aluminum article in an electrolytic bath consisting essentially of from about 20% to 60%, by Weight, of 100% sulfuric acid and the balance water, anodically etching the immersed aluminum article by passing therethrough a direct current of sufiicient density to provide a current density of frOm about 50 to 75 amperes per square foot for from about 5 to 15 seconds, rinsing the etched aluminum article in clean water, immersing the etched aluminum article in an aqueous alkali metal zincate solution for from about 15 seconds to 2 minutes to deposit thereon a substantially non-porous layer of zinc, rinsing the zinc coated aluminum article in clean water, electrodepositing a silver strike on the applied zinc layer from a strike bath essentially composed of an aqueous solution of silver cyanide and sodium cyanide, said silver strike bath consisting essentially of from about 1 gram to 5 grams of silver cyanide and from about to grams of sodium cyanide per liter of bath, the balance being water, and electrodepositing a silver plate over said silver strike from an aqueous bath comprising silver cyanide for a period of time sutficient to apply the desired thickness of silver plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,147,718 Hall July 27, 1915 2,040,618 Mason et al May 12, 1936 2,142,564- Korpiun Jan. 3, 1939 2,336,846 Clark Dec. 14, 1943 2,650,901 Van der Horst Sept. 1, 1953 2,708,655 Turner May 17, 1955 2,721,835 Axtell Oct. 25, 1955 2,739,932 Forestek Mar. 27, 1956 2,791,553 Connor et al May 7, 1957 2,879,210 Howard Mar. 24, 1959 2,888,387 Wasserman May 26, 1959 OTHER REFERENCES Modern Electroplating, edited by A. G. Gray, John Wiley and Sons Inc., New York, 1953, pages 374-75. 

1. IN THE METHOD OF SILVER PLATING AN ALUMINUM ARTICLE THE STEPS WHICH COMPRISE IMMERSING THE ALUMINUM ARTICLE IN AN ELECTROLYTE BATH CONSISTING ESSENTIALLY OF 20% TO 60%, BY WEIGHT, OF A MINERAL ACID OF 100% CONCENTRATION SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND NITRIC ACID AND THE BALANCE WATER, ANODICALLY ETCHING THE IMMERSED ALUMINUM ARTICLE BY PASSING THERETHROUGH A DIRECT CURRENT OF SUFFICIENT DENSITY TO PROVIDE A CURRENT DENSITY OF FROM ABOUT 50 TO 75 AMPERES PER SQUARE FOOT FOR ABOUT 5 TO 15 SECONDS, RINSING THHE ETCHED ALUMINUM ARTICLES IN CLEAN WATER, IMMERSING THE ETCHED ALUMINUM ARTICLE IN AN AQUEOUS ALKALI METAL ZINCATE SOLUTION FOR FROM ABOUT 15 SECONDS TO 2 MINUTES TO DEPOSIT THEREON A SUBSTANTIALLY NON-POROUS LAYER OF ZINC, RINSING THE ZINC COATED ARTICLE IN CLEAN WATER, ELECTRODEPOSITING A SILVER STRIKE OVER THE APPLIED ZINC LAYER FROM A STRIKE BATH ESSENTIALLY COMPOSED OF AN AQUEOUS SOLUTIONS SILVER CYANID AND SODIUM CYANIDE, SAID SILVER STRIKE BATH CONSISTING ESSENTIALLY OF FROM ABOUT 1 GRAM TO 5 GRAMS OF SILVER CYANIDE AND FROM ABOUT 65 TO 90 GRAMS OF SODIUM CYANIDE PER LITER OF BATH, THE BALANCE BEING WATER, AND ELECTRODEPOSITING A SILVER PLATE OVER SAID SILVER STRIKE WHEREBY THERE IS PRODUCED AN ALUMINUM ARTICLE HAVING A HIGHLY ADHERENT SILVER PLATE APPLIED THERETO. 